1. Field of the Invention
The present invention relates to semiconductor devices. In particular, the present invention relates to power supply circuits and methods for driving the power supply circuits.
Note that in this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics. An electrooptic device, a display device (including a light-emitting display device), a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A direct current-to-direct current (DC-DC) converter is known as one of power supply circuits that change output power depending on variations in circuit loads to output stable power.
DC-DC converters are often used in power supply circuits that supply stable power to light-emitting devices including light-emitting diode (LED) elements, organic electroluminescence (EL) elements, or the like. DC-DC converters are adapted to a variety of other uses. For example, DC-DC converters are used in control circuits for controlling charging and discharging of electric storage devices or power supply circuits for supplying power to drive motors.
Patent Document 1 discloses a lighting apparatus including a DC-DC converter and an LED element.
For example, the emission luminance of a current drive light-emitting element such as an LED element or an organic EL element changes depending on current flowing through the element. Thus, in the case where a DC-DC converter is used in a light-emitting device including such a light-emitting element, current flowing through the element is controlled to be always constant in order to stabilize emission luminance.
FIG. 10 illustrates a structure example of a conventional DC-DC converter.
The DC-DC converter in FIG. 10 is a step-down DC-DC converter outputting voltage that is lower than input voltage. In FIG. 10, the DC-DC converter includes two switches (a switch 12 and a switch 13) connected in series, a controller 11 controlling the operation of each switch, a coil 14, a current detection resistor 15, a capacitor 16, and a load 17.
Here, the current detection resistor 15 is provided to detect the value of current flowing through the load 17. As illustrated in FIG. 10, by connecting the current detection resistor 15 in series with the load 17, it is possible to measure current flowing through the load 17 from a potential difference (voltage) across the current detection resistor 15.
Voltage applied to the current detection resistor 15 is input to the controller 11, and the operation of the two switches is controlled so that the voltage is at a desired value, that is, current flowing through the load 17 is constant. Thus, stable power can be output.
In this manner, conventional DC-DC converters often employ a method for detecting and controlling current flowing through a load (hereinafter also referred to as a current control method).